This invention relates to a method for the preparation of polycarbonate. More particularly the method relates to a method of preparing polycarbonate by the melt reaction of at least one dihydroxy aromatic compound with at least one diaryl carbonate, said melt reaction being mediated by a transesterification catalyst and optionally a co-catalyst said transesterification catalyst comprising at least one tetraarylphosphonium compound and said co-catalyst comprising an alkali metal hydroxide said product polycarbonate comprising less than 1000 parts per million Fries product.
Conventionally, polycarbonate is prepared by the reaction of a dihydroxy aromatic compound such as bisphenol A with phosgene in the presence of an aqueous phase comprising an acid acceptor such as sodium hydroxide and an organic solvent such as dichloromethane. Typically, a phase transfer catalyst, such as a quaternary ammonium compound or a low molecular weight tertiary amine, such as triethylamine is added to the aqueous phase to enhance the polymerization rate. This synthetic method is commonly known as the xe2x80x9cinterfacialxe2x80x9d method for preparing polycarbonate.
The interfacial method for making polycarbonate has several inherent disadvantages. First it is a disadvantage to operate a process that requires phosgene as a reactant due to obvious safety concerns. Second it is a disadvantage to operate a process that requires using large amounts of an organic solvent because elaborate precautions must be taken to prevent adventitious release of the volatile solvent into the environment. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process is prone to having inconsistent color, higher levels of particulates, and higher chlorine content, which can cause corrosion.
More recently polycarbonate has been prepared on a commercial scale in a solventless process involving the transesterification reaction between a dihydroxy aromatic compound (e.g. bisphenol A) and a diaryl carbonate (e.g., diphenyl carbonate) in the presence of a transesterification catalyst. This reaction is performed in a molten state in the absence of solvent, and is driven to completion by mixing the reactants under reduced pressure and high temperature with simultaneous distillation of the phenol by-product produced by the reaction. This method of preparing polycarbonate is referred to as the xe2x80x9cmeltxe2x80x9d process. In some respects the melt process is superior to the interfacial method because it does not employ phosgene, it does not require a solvent, and it uses less equipment. Moreover, the polycarbonate produced by the melt process does not contain chlorine contamination from the reactants, has lower particulate levels, and has a more consistent color. Therefore it is highly desirable to use the melt process when making polycarbonate in commercial manufacturing processes.
A wide variety of transesterification catalysts have been evaluated for use in the preparation of polycarbonate using the melt process. Quaternary ammonium salts and alkali metal hydroxides, in particular sodium hydroxide, have proven to be particularly effective as transesterification catalysts. However, while alkali metal hydroxides are useful polymerization catalysts, they are also known to promote Fries reaction along the growing polycarbonate chains which results in the production of branched polycarbonate products. The presence of branching sites within a polycarbonate chain can cause changes in the melt flow behavior of the polycarbonate, which can lead to difficulties in processing.
It would be desirable, therefore, to develop a method for conducting melt polymerization reactions to provide product polycarbonates having high molecular weight while minimizing undesirable reactions, such as the Fries reaction.
In one aspect, the present invention provides a method of preparing polycarbonate, said method comprising Step (A) oligomerising in the presence of a catalyst, at least one diaryl carbonate and at least one dihydroxyaromatic compound at a temperature in a range between about 220 and about 280xc2x0 C. and a pressure in a range between about 180 mbar and about 20 mbar, said catalyst comprising a tetraaryl phosphonium compound and optionally a co-catalyst, to provide an oligomeric polycarbonate having a number average molecular weight in a range between about 1000 and about 7500 daltons and in a second step, Step (B) heating the oligomeric a polycarbonate formed in step (A) at a temperature in a range between about 280 and about 310xc2x0 C. and at pressure in a range between about 15 mbar and about and about 0.1 mbar to provide a polycarbonate having a number average molecular weight between about 15000 daltons and about 50,000 daltons, said method comprising less than about 1000 parts per million Fries product.
In a further aspect, the present invention relates to both polycarbonate oligomers and high molecular weight polycarbonates prepared according to the method of the present invention.